The invention relates to devices for microwave transmission between two bodies which are rotatable relative to each other. Such devices are sometimes known as microwave slip rings. Such a device has two circular waveguides arranged on an axis. Each waveguide comprises a receiving and a transmitting aerial and has along its circumference a coupling slot. Two of the four aerials project into the two coupling slots and are movable therein. These two aerials are mechanically coupled together. The two other aerials are fixed to the associated waveguides.
Such a device is disclosed in U.S. Pat. No. 3,419,826. As described in that patent, the device can be used for telephone connections for rotating apartment houses, for a radar aerial, or for an airfield control tower.
Contactless transmission of information is also needed in computer tomography devices. In these devices, X-rays are passed through a patient and are detected by a plurality of X-ray detectors. The detectors and the X-ray source are fitted on a member which rotates around the patient. The detector signals from the rotating member must then be transmitted to a stationary member so that they can be processed by a computer. When the radiator rotates more than once around the patient, transmission of the detector signals via cables is no longer possible. The information can then be transmitted by the device described above when the transmission rate or the data stream is sufficiently large.
In the prior art arrangement, one waveguide is semicircular and is closed at both ends by attenuators. The other waveguide is a closed ring. An attenuator is provided in the closed ring to prevent a wave from circulating more than once in the ring. A changeover switch is provided which, depending on the positions of the receiving aerials projecting into the coupling slot, renders either the one or the other receiving aerial operative.
In certain circumstances in the prior art device, an electromagnetic wave must travel substantially completely around the waveguide ring before it arrives at the receiving aerial. In the semicircular waveguide, the maximum wave path is only half as long. As a result, sudden changes are produced in the propagation time when the receiving aerials are changed over. These sudden changes considerably impede continuous signal transmission at a high transmission rate.
The sudden changes in the propagation time do not occur in the embodiment shown in FIG. 6 of U.S. Pat. No. 3,419,826. However, this embodiment requires two changeover switches which alternately connect one out of two transmitting aerials to one out of two receiver aerials. The changeover switches must always be changed over at the correct moment, which requires complicated and expensive electronic circuitry.